A typical molten metal facility includes a furnace with a pump for moving molten metal. During the processing of molten metals, such as aluminum and zinc, the molten metal is normally continuously circulated through the furnace by a centrifugal circulation pump to equalize the temperature of the molten bath. These pumps contain a rotating impeller that draws in and accelerates the molten metal creating a laminar-type flow within the furnace.
To transfer the molten metal out of the furnace, typically for casting the metal, a separate centrifugal transfer pump is used to elevate the metal up through a discharge conduit that runs up and out of the furnace. As shown in FIG. 1, a typical prior art transfer pump includes a base 5, two to three support posts 6 (only one shown), a shaft-mounted impeller 7 located within a pumping chamber or volute 5a in the base 5, a motor 8 and motor mount 9 which turn the impeller, bearings 10 that support the rotating impeller (and shaft), and a riser tube or conduit 11 located at the outlet of the base. The riser 11 is provided to allow the metal to lift upward over the sill edge of the furnace in order to transfer some of the molten metal 12 out of the furnace into ladles or molds.
A well-known problem with previous transfer pumps, however, is that the relatively narrow riser tube 11 becomes clogged as small droplets of the molten metal accumulate in the riser each time the pump stops transferring and the metal stops flowing through the riser. Initially, the metal accumulates in the porosity of the riser tube material (typically graphite or ceramic) and then continues to build upon the hardened metal/dross until a clog 13 occurs. As a result of this problem, furnace operators must frequently replace the transfer pump's riser tube as they are too narrow to effectively clean. This replacement typically requires the furnace to be shut down for an extended period to remove the clogged riser tube.
Several treatments have been used to alleviate this riser-clogging in transfer pumps. Including impregnating, coating, and inert gas pressurization of the riser to reduce the build-up within the tube. Another method pump manufacturers employ is to simply increase the diameter of the riser to delay the blockage. These treatments have varying degrees of success, but still only delay the inevitable clogging of the riser.
Another common operation in a molten metal facility is to add scrap metal, typically metal working remnants or chips, to the molten bath within a furnace. The heat of the bath melts the chips. Currently, the added chips are simply allowed to fall into the bath or may be mixed into the molten metal by a circulation pump. The current process(es), however, is not effective to fully immerse the solid chips into the molten bath resulting in a longer melt time. As shown in FIG. 2, prior art systems utilizing a dedicated mixing pump 14 directs molten metal 12 into a vessel 15 resulting in a nearly free vortex 16 to be formed. Scrap metal, such as chips 17, are deposited into the vortex 16 to mix and melt the chips 17 within the molten metal bath 12. As will be discussed in greater detail below, these nearly free vortex-based systems do not provide sufficient residence times within the bath to efficiently melt and mix the newly deposited chips into the bath. A nearly free vortex 16, such as the type formed by prior art systems are governed by the following equations ω=Cr−2, Vr=C, P/γ=C/2gr+h, where ω is the angular velocity, V is the peripheral velocity, P/γ is the pressure distribution (pressure energy) and h is the static energy. Then the maximum velocity would be at the center axis of the vortex, expelling the chips upward and outwards. Consequently, any metal particles introduced therein will float at the top of the metal and exit without being melted at the top exit outlet, generating a large amount of dross instead of liquid metal. The resulting shape of vortex 16 is shown in FIG. 12.
Presently, molten metal facilities have limited furnace footprints with relatively small pump wells and charge wells. The limited space available typically prevents furnace operators from having permanently installed transfer pumps and/or mixer/pre-melter systems within a furnace (in addition to the recirculation pump). There is therefore a need for a system that can combine two or more of the transfer/pre-melt/recirculation processes within a single pump.
Another drawback of conventional molten metal pumps is the highly inefficient use of very large/high horsepower motors which are stepped down in velocity electronically with a frequency converter that maintains the torque constant, thus reducing the output horsepower when running the equipment at a safe RPM. The present invention provides for a mechanical gear box which provides for the desired reduction in RPMs, while boosting the torque and permitting the motor to operate at or near its optimal speeds to further increase efficiency. The gear box necessitates another improvement to existing molten metal pumps to avoid the undesirable transfer of heat from the molten bath into the gear train and pump motor. This further improvement is a coupling that operates as a thermal barrier between the drive shaft that is submerged within the bath to rotate the pump impeller and the upper portion of the shaft that is driven by the gear train.
In view of the current inefficient use of molten metal transfer and mixing pumps, there is a need for a molten metal pump that overcomes all of the above-indicated drawbacks.
The present invention provides a molten metal pump including an elongated body or vessel having an elongated bowl or tube that terminates in a curved bottom end. A centrifugal impeller is seated in an inlet opening formed in the center of the bottom end. The shape of the vessel's bottom end provides a smooth upward transition for metal ejected from the impeller to the inner walls of the tube. The rotation of the impeller centered in the curved lower walls results in the ejected flow of molten metal to create an uplifting vortex which climbs the inner walls of the vessel to an outlet opening in an upper portion wall. The pump is preferably a hybrid-drag turbine type disclosed in my U.S. Pat. No. 8,033,792 which is incorporated herein.
Further, the present molten metal pump includes a second centrifugal impeller mounted coaxially to the vortex lifting impeller. The second centrifugal impeller is a recirculation pump and is preferably a turbine impeller such as the ones disclosed in my U.S. Pat. No. 7,896,617 (turbine) which is incorporated herein and which provides a very high outlet peripheral velocity.
The vessel's shape (i.e., the curvature of the inner wall and bottom end) is a function of the type of vortex required by the particular application. Particularly, I have determined that the optimum vortex for transfer-only applications maintains a constant angular velocity using an internally curve-shaped vessel that concurs with the following equations ω=CTE (constant, i.e., ω=Cr0 with Vr−1=C), P/γ=Cr2/2g+h. The constant angular velocity of the liquid metal moves like a solid, while twisting and turning upwards in the vessel without each molecular layer sliding with respect to the adjacent layer minimizing the possibility of turbulence, loss of heat and viscous windage losses.
Further, I have determined that the optimum vortex for a mixing/pre-melting application requires an internally curve-shaped vessel when flows greater than 1500 GPM are required follow the equations ω=Cr, Vr−2=C, P/γ=Cr4/4g+h. A curve that follows the equations ω=Cr1/2 with Vr−3/2=C, P/γ=Cr3/3g+h will suffice for lower flow rates. The vortex created in a mixing/pre-melting application should be a highly forced or super forced vortex to assure the penetration of the particles of added material into the matrix of partially combined material. To ensure adequate churning or slipping between adjacent molecular layers the angular velocity is higher toward the periphery of the vortex. At higher flow rates a hyperbolic-shaped vessel creates a super forced vortex requiring a hybrid-drag turbine type impeller such as the type disclosed in my U.S. Pat. No. 8,033,792. At lower flow rates a turbine impeller may be used to generate the flows and velocities necessary, such as the type disclosed in my U.S. Pat. No. 7,896,617.
The present invention further provides an improved system for transmitting the requisite torque to the combined impellers. The present power transmission includes a mechanical gear train which reduces the revolutions per minute from the pump's electric motor, while also boosting the torque being applied along the output shaft.
The present invention still further provides a self cooling thermal barrier coupling which transmits the torque from the motor/gear box, while limiting the conduction of heat from the molten metal bath to the motor and gear box.
It is an advantage of the present invention to provide a pump which creates a forced, highly forced or super forced vortex of molten metal within a generally vertical tube body of the pump to lift the whirling molten metal for transferring, mixing, and/or pre-melting applications.
It is another advantage of the present invention that the lifting cavity has a relatively large internal diameter allowing the inner walls to be readily accessed for cleaning and removal of accumulated metal and dross. Preferably, the vessel internal diameter being between 1.5 to 4 times the impeller outside diameter.
It is still another advantage of the present invention that two coaxial impellers are driven by a common drive motor-shaft design to simultaneously provide the lifting vortex flow and the recirculation flow. An upper impeller being mounted within the tubular lifting vortex cavity, while the lower impeller is mounted within a volute to recirculate a bath of molten metal.
It is still yet another advantage of the present invention that the lifting vortex cavity has a curved shape and size that complements the intended vortex formed therein. That is, in a transfer application, the lifting cavity has a particular curvature shape, while in a pre-melting/mixing application the lifting vortex cavity has at least a third degree curved shape as described above (e.g., has a pressure distribution following P/γ=C2r3/3g+h or P/γ=C2r4/4g+h).
An advantage of the present invention over prior art transfer-type pumps is that the present invention eliminates the support posts, riser tube, and one impeller bearing thereby reducing the complexity of the pump system and reducing the number of components subject to deterioration due to the molten metal environment and which must eventually be replaced.
It is an additional advantage of the present invention when mixing or pre-melting to provide an upper impeller having a plate with a plurality of radial vanes facing into the tubular body. When metal scrap chips are inserted into the pump's tubular cavity, the plurality of radial vanes on the upper impeller causes the metal chips to be directed radially outwardly into the pump-generated uplifting vortex of molten metal. The rotational velocity of the impeller causes the chips to further penetrate the surface of the vortex to fully immerse the chips within the molten metal.
It is yet another advantage of the present invention that the dual impellers are driven by the motor through a mechanical gear train which increases the torque transmitted to the output shaft and permits the motor to run near its optimal operating speed by reducing the rotational speed of the output shaft to a desired amount.
These and other objects, features and advantages of the present invention will become apparent from the following description when viewed in accordance with the accompanying drawings.